Test and attempt implementing logarithmic depth buffer

[Riot]

If applicable, then test reducing overlap gaps for roads over terrain

[Riot]

implementing logarithmic depth buffer for meatspace - resolves #83, thanks passivefriendly :)

→ <<cset fb0b81dca894>>

[Riot]

tweaking logarithmic depth buffer and adding basic fragment fog - re #83, resolves #73

→ <<cset 0e8cce6bfc1d>>

[Riot]

correct picking with both normal and logarithmic depth buffer, re #83

→ <<cset 079ec4ca82d6>>

[Riot]

optimising logarithmic depth buffer vertex data collection - re #83

→ <<cset 46ea02eb45d3>>

[Riot]

further logarithmic depth buffer optimisations, re #83

→ <<cset c77971fdf3ec>>

[Riot]

pre-computing logarithmic depth buffer constant, re #83

→ <<cset 5ccefffc3de6>>

[Riot]

switching logarithmic depth buffer to be per-vertex rather than per-fragment for considerable speedup, at the cost of potential accuracy with camera-crossing polygons - re #83

→ <<cset 8288c678e497>>